The present invention relates to innovative polyisocyanate mixtures comprising cycloaliphatic structural elements and containing exclusively linear-aliphatically attached free isocyanate groups, to a process for preparing them and to their use as a starting component in the production of polyurethane plastics, more particularly as a crosslinker component in polyurethane coating materials and coatings.
The outstanding technological properties of two-component polyurethane (2K-PUR) coating materials have given them importance for a multiplicity of different applications. As crosslinker components for light-stable, non-yellowing 2K-PUR coating materials and coatings, it is common to use polyisocyanates based on linear-aliphatic and/or cycloaliphatic diisocyanates.
In the most applications of 2K-PUR coating materials, polyisocyanates based on 1,6-diisocyanatohexane (HDI) are presently employed. Even at low temperatures, these polyisocyanates lead to coatings having good resistance to chemical and mechanical exposure, but in many cases exhibit an inadequate drying rate and comparatively low ultimate hardnesses.
The same is true for the use of special high-functionality HDI polyisocyanates, examples being high-functionality HDI trimers, which are often recommended as crosslinker components for the formulation of quick-drying 2K-PUR coating materials (see, for example, product information on Tolonate® XFD 90 B, Rhodia PCS). Although it is indeed possible using such crosslinkers to raise the drying rate in relation to standard crosslinkers, a higher ultimate hardness on the part of the coating films is not obtained.
With respect to achievable film hardness, distinct advantages are possessed by polyisocyanate crosslinkers based on cycloaliphatic diisocyanates, such as isophorone diisocyanate (IPDI), for example. While polyisocyanates based on linear aliphatic diisocyanates are distinguished in particular by low viscosities and produce coating films of high elasticity, cycloaliphatic polyisocyanates in solvent-free form are highly viscous to solid products which lead to rapid physical drying of a coating preparation and yield coatings of very high hardness.
HDI polyisocyanates are therefore used frequently in combination with corresponding cycloaliphatic polyisocyanates, examples being those based on IPDI (see, for example, U. Meier-Westhues, Polyurethane-Lacke, Kleb-und Dichtstoffe, Vincentz Network 2007, p. 166). In this way the drying of the coating films, and more particularly the development of hardness, can be considerably accelerated. For complete chemical crosslinking, however, cycloaliphatic polyisocyanates, on account of the lower reactivity of their isocyanate groups, require temperatures in the region of 100° C. or above. In the case of room temperature or slightly forced drying (about 60° C.), the coating films obtained are indeed rapidly touch-dry and hard, but exhibit lower solvent resistance and chemical resistance than coatings crosslinked exclusively using HDI polyisocyanates.
It was an object of the present invention, therefore, to provide new polyisocyanates which are suitable for all fields of application of aliphatic polyisocyanates, and more particularly as crosslinker components for 2K polyurethane coating materials, but which are not hampered by the disadvantages of the known systems. These new polyisocyanate crosslinkers ought to permit the formulation of quick-drying coating systems which at the same time cure even at low temperatures to give hard, solvent-resistant and chemical-resistant coatings.